1. Field of the Invention
This invention relates to a single phase article and to a multiphase composite and to a method for producing the same.
2. Description of the Prior Art
Composite produces having multiphases of matrix metal and a hardening phase are used in various applications requiring hard, wear-resistant properties. The composites comprise a metal matrix, which may be for example, iron, nickel, or cobalt, with a hard-phase nonmetallic dispersion therein of, for example, carbides, nitrides, oxynitrides or industrial diamonds.
Tungsten carbide-cobalt composites are one significant example of composites of this type and the production thereof typifies the conventional practices used for the manufacture of these composites.
The manufacturing process consists of synthesis of the pure carbide and metal powders, blending of the carbide and metal powders to form a composite powder, consolidation of the composite powder to produce a "green" compact of intermediate density and, finally, liquid phase sintering of the compact to achieve substantially full density.
Preparation of the tungsten carbide powder conventionally comprises heating a metallic tungsten powder with a source of carbon, such as carbon black, in a vacuum at temperatures on the order of 1350.degree. to 1600.degree. C. The resulting coarse tungsten carbide product is crushed and milled to the desired particle size distribution, as by conventional ball milling, high-energy vibratory milling or attritor milling. The tungsten carbide powders so produced are then mixed with coarse cobalt powder typically within the size range of 40 to 50 microns. The cobalt powders are obtained for example by the hydrogen reduction of cobalt oxide at temperatures of about 800.degree. C. Ball milling is employed to obtain an intimate mixing of the powders and a thorough coating of the tungsten carbide particles with cobalt prior to initial consolidation to form an intermediate density compact.
Milling of the tungsten carbide-cobalt powder mixtures is usually performed in carbide-lined mills using tungsten carbide balls in an organic liquid to limit oxidation and minimize contamination of the mixture during the milling process. Organic lubricants, such as paraffins, are added to the powder mixtures incident to milling to facilitate physical consolidation of the resulting composite powder mixtures. Prior to consolidation, the volatile organic liquid is removed from the powders by evaporation in for example hot flowing nitrogen gas and the resulting lubricated powders are cold compacted to form the intermediate density compact for subsequent sintering.
Prior to high-temperature, liquid-phase sintering, the compact is subjected to a presintering treatment to eliminate the lubricant and provide sufficient "green strength" so that the intermediate product may be machined to the desired final shape. Presintering is usually performed in flowing hydrogen gas to aid in the reduction of any residual surface oxides and promote metal-to-carbide wetting. Final high temperature sintering is typically performed in a vacuum at temperatures above about 1320.degree. C. for up to 150 hours with the compact being imbedded in graphite powder or stacked in graphite lined vacuum furnaces during this heating operation. In applications where optimum fracture toughness is required, hot isostatic pressing at temperatures close to the liquid phase sintering temperature is employed followed by liquid phase sintering to eliminate any residual microporosity.
With this conventional practice, problems are encountered both in the synthesis and the blending of the powders. Specifically, kinetic limitations in the synthesis of the components require processing at high temperature for long periods of time. In addition, control of carbon content is difficult. Likewise, compositional control is impaired by the introduction of impurities during the mechanical processing of the composite powders and primarily during the required milling operation. Likewise, the long time necessary for achieving microstructural control and homogenization during milling adds significantly to the overall processing costs. Also, microstructural control from the standpoint of achieving desired hard-phase distributions is difficult. Specifically, in various applications extremely fine particle dispersions of the hardening phase within a metal matrix is desired to enhance the combination of hardness, wear resistance and toughness.